Welding applications, such as pipe welding, often require high currents and use several arcs created by tandem electrodes. Such tandem welding systems are described, for example, in Stava 6,207,929 and Stava 6,291,798. Houston 6,472,634 discloses the concept of a single AC arc welding cell for each electrode wherein the cell itself includes one or more paralleled power supplies each of which has its own switching network. The output of the switching network is then combined to drive the electrode. The power supplies can be paralleled to build a high current input to each of several electrodes used in a tandem welding operation.
Stava 6,291,798 discloses a series of tandem electrodes movable along a welding path to lay successive welding beads in the space between the edges of a rolled pipe or the ends of two adjacent pipe sections. The individual AC waveforms are suitably created by a number of current pulses occurring at a frequency of at least 18 kHz with a magnitude of each current pulse controlled by a wave shaper. This technology dates back to Blankenship 5,278,390. In Stava 6,207,929, the frequency of the AC current at adjacent tandem electrodes is adjusted to prevent magnetic interference.
Computation of the heat input in the case of waveform controlled welding is complicated by the complex shape of the voltage and current waveforms. A product of the rms current times the rms voltage provides a measure of the heat input, but such a computation does not take into account the precise shape of the waveform and possible phase offsets between the voltage and current. A generally more accurate method for computing heat input in waveform controlled welding is described in Hsu, U.S. published application 2003-0071024 A1.
One difficulty with tandem welding is characterizing and monitoring the quality of the tandem weld. Analysis of tandem arc welding is complicated due to the use of multiple electrode wires for depositing metal simultaneously but at spatially separated positions. The electrode wires of the tandem electrodes may have different wire diameters. The wire feed speed of each electrode may be independently dynamically adjusted for each electrode to control the arc length or other welding characteristics. In some tandem arc welding applications, a combination of electrodes operating using d.c. current and a.c. current may be employed, for example to reduce interference between the electrodes. Still further, the voltage and/or current of each electrode may be independently controlled.
At a given location of the weld, each electrode in general contributes weld bead material at different times during the weld process. The metal deposition rate, heat input, and other welding parameters for that location depend upon the combined effect of the several electrodes of the tandem arrangement, but the contributions of the several electrodes are separated in time.
The present invention contemplates an improved apparatus and method that overcomes the above-mentioned limitations and others.